Raw alumina is mined and used for any number of purposes with alumina as one of the compounds. Various forms of alumina are known with varying water content even in the raw/synthesized stages. This serves as the basic feed stock for a number of products. One particular process step that is important to the present disclosure is the conversion of alumina in the processed form into other products. For example, alumina if appropriately heated to the level of sintering forms particles of a controlled size and weight which have substantial use in abrasives, abrasive wheels, grinding materials and the like. Attempts have been made at processing the green alumina, i.e., mined alumina having hydrate water associated with it in a variety of particle sizes; these attempts have been successful provided the alumina can be placed in a suitable powdered form preliminary to sintering. Sintering in a heated oven has a limited level of success. Long time intervals are normally required to achieve the excessive temperatures necessary which first drives off the water and volatiles prior to sintering. After the water and other volatile parts are vaporized, the alumina has to be rearranged (in a technical sense), that is it, must be converted from the initial state into a suitable state for sintering in this fashion. That has limited success and can be done, but at a cost. One cost is the fuel consumption required for the heating process. Another cost is the occasional loss by converting the alumina into a undesired chemical forms.
Recently, it has been discovered that alumina can be microwave sintered. This has had a measure of success also. For example, recently issued U.S. Pat. No. 5,858,037 of the present applicant sets out an alumina processing procedure utilizing alumina transported in a hollow upstanding tube. The microwave procedure provides good control of the process at reduced utility cost. There is, however, a negative to this. One negative relates primarily to the shrinkage which occurs when heated to higher temperatures. There is the initial loss of volatile matters, leaving porosity inside the material followed by rearrangement of molecules change crystalline strucuture, phase and densification. When actually done in an upstanding circular tube, there is a tendency of the alumina material to be sufficiently and appropriately rearranged into the crystalline structure while material shrinkage occurs. This shifts the physical location of the particles as they are fed through the hollow cylindrical tube. Where the tube passes through the immediate vicinity of a microwave chamber where heating does occur, the concentration of the heat in the center of the elongate cylindrical mass is different in comparison with the particles at the exterior. When shrinkage occurs, the change in the mass with the resultant shrinkage changes the velocity in the tube. Particles near the centerline axis and midportions of the tube travel through the heating zone at a specified velocity. With shrinkage, however, the tube which is full above will then not be so full below the heating zone. With this volumetric loss, the standing column of alumina particles in the tube will shrink around the periphery, thereby pulling away from the side walls of the tube. Effectively, this creates a side wall gap which is below the heating zone. When that gap is formed, material funnels down into the gap along the side wall of the tube. As this material falls through the gap and refills near the bottom of the tube, it changes the intended transit time for the particles that fall into that gap. Therefore, a particle which is located at the centerline axis of the tube will proceed through the heating zone at the calculated velocity. This velocity is determined by the rate at which the particles are removed from the bottom of the tube while new particles are added at the top. Next to the tube wall, particles fall into the gap and do not spend an adequate interval in the heating zone. They are "undercooked" so to speak. While the particles at the center are properly heated at the right temperature in the right interval and form highly desirable sintered alumina particles finding application in other procedures, those alumina particles near the side wall will fall through so rapidly that they are undercooked and are therefore still green, not fully processed. They do not achieve the desired crystalline structure. They may contain some volatile matter and/or would lack in density/hardness requisite for abrasive job application. They may also still lack the hardness requisite for abrasive job applications. Finally, they still are mixed in with the other particles which are adequately processed. There might be some remote chance for sintering of these unsintered or under processed particles as they escape from the micorwave high temperature zone. But this normally is not sufficient enough to accomplish post microwave processing of green particles into the desired form which is accomplished primarily in the micorwave zone. In effect, when the process is not efficient and creates a separation problem, it adds a postmanufacturing step. That separation problem requires the separation of the processed particles which then become highly desirable very hard alumina particles for use in abrasive wheels and the like; the other alumina particles represent a waste portion in the manufacturing process. This waste portion has to be dealt with so that they do not continue to pose a problem.
The present disclosure sets out an improved alumina microwaving process mechanism and procedure. In this, alumina particles are fed in through a chute into a tube. The tube is set at an angle which will be discussed. The tube is rotated at a slow rate of speed to assure that the particles in it are tumbled, moved against the side wall and are relatively firmly packed. When this is done, the rotating tube then permits the alumina particles to move into and out of a zone. So to speak, the zone is maintained in a desired microwave energy field to assure appropriate cooking. By apt choice of the length of the tube, the diameter of the tube, the rate of removal and therefore the rate of adding particles to the tube and the microwave energy levels chosen, suitable microwave sintering can be accomplished. At the end of the process, it is able to discharge all the particles as the finished alumina sintered particles. They are not mixed with half cooked alumina particles which may still have water in the molecules, and the alumina is converted into the desired crystalline form.